1. Field of the Invention
The present invention relates to a system for controlling the gear ratio of an automatic transmission disposed on the output side of a prime mover and, more particularly, to a speed change control system for an automatic transmission, for selectively switching control modes to control the gear ratio.
2. Related Art
Generally, an automatic transmission having a gear transmission mechanism is constructed to include: frictional engagement elements to be applied/released for switching torque transmission lines of the gear transmission mechanism; a hydraulic control unit for controlling an oil pressure to be fed to and discharged from the frictional engagement elements; and an electronic control unit for controlling the hydraulic control unit. This electronic control unit is stored with a shift map for providing references for deciding the switching of a gear ratio of the automatic transmission. This shift map determines an upshift line or a downshift line, i.e., shift lines for controlling the gear ratio of the automatic transmission with parameters indicating the running state of a vehicle, such as a vehicle speed or an accelerator depression or opening.
When a signal indicating the running state of the vehicle is inputted to the electronic control unit, moreover, a speed change is decided on the basis of the input signal and the shift map. When the decision to switch the gear ratio of the automatic transmission is made, a shift signal is outputted from the electronic control unit so that the frictional engagement elements are applied/released to execute the speed change by the hydraulic control unit.
On the other hand, the shift map determines the gear ratio in accordance with the running state, as represented by the vehicle speed or the accelerator opening, so that the characteristics of the vehicle change in various ways according to the manner to set the region of the gear ratio. If the region of a higher gear ratio is set so far to a running state of a relatively high vehicle speed, more specifically, the power performance improves. If the region of a low gear ratio is set to a running state of a relatively low vehicle speed, on the other hand, the increase in the speed of the prime mover can be suppressed to take an advantage for improving the fuel economy. In the prior art, therefore, there is known a technique, in which there are prepared: a shift map constructed to have characteristics thinking much of the fuel economy of the prime mover; and a shift map constructed to have characteristics thinking much of the power performance of the vehicle, so that the shift maps are changed by the manual operation of the driver. In the prior art, moreover, there is proposed a technique for switching the shift maps of different control contents automatically according to the change in the running load on the vehicle.
One example of the speed change control system for the automatic transmission, for switching the maps of different speed change control contents automatically according to the change in the running load on the vehicle has been disclosed in Japanese Patent Application Laid-Open No. 6 -147304 or 11-51169. In the speed change control system disclosed in Japanese Patent Application Laid-Open No. 6-147304, the running load on the vehicle is estimated on the basis of the vehicle weight, the output torque of the engine and the vehicle acceleration, and a shift map is selected on the basis of the estimation result of the running load on the vehicle. The contents of the shift map thus selected are different in the speed change control contents for either an upshift or a downshift. For the upshift, for example, the speed change control contents are such that the upshift line is set toward the higher vehicle speed side for the larger vehicle weight and the heavier load. For the downshift, on the other hand, the speed change control contents are such that the downshift line is moved toward the lower degree of throttle opening for the heavier load. At the time of a heavy load and a large vehicle weight, therefore, a relatively high gear ratio can be easily selected to improve the power performance of the vehicle.
On the other hand, the speed change control system, as disclosed in Japanese Patent Application Laid-Open No. 11-51169, is applied to a vehicle such as a tractor for pulling a trailer. Specifically, the speed control system of this Laid-Open employs a shift map of such speed change characteristics that a relatively low gear ratio is easily selected at a light-load time when the tractor runs by itself. In a heavy-load state where the tractor runs while pulling the trailer, on the other hand, there is employed a shift map having contents in which a relatively high gear ratio is fixed. Thus, shift maps of different kinds are automatically interchanged on the basis of whether or not the tractor pulls the trailer, to take advantages in the improvements in the drivability and in the fuel economy of the prime mover.
In the control system of this kind for the automatic transmission, however, the shift map to be used in the heavy-load running state, i.e., with the accelerator being fully opened, is prepared such that the upshift line is intrinsically set on the higher speed side to set the higher gear ratio easily. Considering the delay, i.e., the response delay of the oil pressure till the speed change is actually started after the output of a shift signal, more specifically, the shift map is set so that the engine speed at the instant when the speed change is actually started may become a value near the upper limit of an allowable speed. Where the upshift line of the shift map is set to a higher vehicle speed side so as to meet the heavy-load demand, as disclosed in the Laid-Open, therefore, the vehicle speed may rise with the high gear ratio to cause the so-called xe2x80x9cover-rev statexe2x80x9d in which the engine speed exceeds the allowable value to reach the red zone.
On the other hand, there can be conceived a control, in which shift maps of different vehicle speeds for setting the upshift line are prepared for the non-pulling state and the pulling state and are interchanged by a switching operation. If this switching operation is mistaken to employ the shift map corresponding to the tractive state in the non-pulling state, however, the upshift is not effected till the engine speed exceeds a predetermined value, and the over-rev state may be invited like the above.
A main object of the invention is to provide a speed change control system for an automatic transmission, for making a speed change control of the automatic transmission of a running vehicle into one suited for a heavy-load running state and for suppressing the rise of the speed of a prime mover to a predetermined value, when the running vehicle is in the heavy-load running state such as when it pulls another vehicle.
According to the invention, there is provided a speed change control system for an automatic transmission, for selectively switching a plurality of speed change control modes of different speed change control contents to control the gear ratio of an automatic transmission disposed on the output side of a prime mover. A first speed change control mode is selected if an output demand for said prime mover is in a predetermined state, and said first speed change control mode is switched to a second speed change control mode if a physical quantity relating to the speed of said prime mover is in a predetermined state when said first speed change control mode is selected.
In the control system of the invention, therefore, the first speed change control mode is selected if the output demand for the prime mover is in the predetermined state, and the first speed change control mode is switched to the second speed change control mode if the physical quantity relating to the speed of the prime mover is in a predetermined state when the first speed change control mode is selected. Therefore, the gear ratio is controlled according to the changing state of the physical quantity, as caused by the change in the running load on the vehicle. Up to the vehicle speed just before the physical quantity relating to the prime mover speed reaches a predetermined large value, for example, the gear ratio of the automatic transmission can be kept at a value as high as possible. As a result, the drive region of the prime mover is so widened that the power performance of the vehicle may be maximized, to avoid shortage of the driving force in the case of a heavy-load run thereby to improve the drivability.
The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read with reference to the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits to the invention.